It is known in the art that some compounds of formula I are useful photographic dyes or dye-forming couplers. Particularly, some of these compounds act as masking couplers. Examples can be found in U.S. Pat. No. 5,466,568 and DE 3530357. The product compounds may be represented by Formula IAr—N═N—R  (I)
The compounds of formula I are most commonly prepared by reacting an aqueous diazonium salt, whose preparation in an acidic aqueous mixture, by reacting an amine compound with a nitrite compound, is well-known in the art (see Practical Organic Chemistry, A. I. Vogel) with a hydrophobic coupler moiety in the presence of pyridine, sometimes in the presence of an organic solvent, as shown in Scheme 1. Examples are U.S. Pat. Nos. 5,622,818; 6,132,943; DE 2706117.

These syntheses suffer from a few drawbacks, first of all in order to promote the azo coupling reaction, they all use pyridine, sometimes in a large excess, which is an expensive, foul-smelling and hazardous volatile organic compound, and which requires treatment of the hazardous waste. Also, they require a multiple-reactor setup, where one reactor is used for the making of the diazonium salt solution, and the second reactor is used for the azo coupling, requiring a transfer of the diazonium salt solution from the first reactor to the second one.
There have been some efforts to use more environmentally-friendly bases, such as some inorganic bases, such as the carbonates, bicarbonates, acetates or hydroxides of alkaline or alkaline-earth metals for making azo compounds. Examples can be found in [Ellwood, M.; Griffiths, J.; Gregory, P. J. C. S. Chem. Comm., 181-183 (1980); Iwamoto, H.; Yoshimura, M.; Sonoda, T.; Kobayashi, H. Bull. Chem. Soc. Jpn, 56, 796-801 (1983); Hashida, Y.; Kubota, K.; Sekiguchi, S. Bull. Chem. Soc. Jpn, 61, 905-909 (1988); Iwamoto, H.; Kobayashi, H.; Murer, P.; Sonoda, T.; Zollinger, H. Bull. Chem. Soc. Jpn, 66, 2590-2602 (1993)]. Examples of actual photographic couplers prepared under these conditions can be found in DD 242050 and examples of azo dyestuff prepared under the same conditions can be found in EP 0028464. However, in all these cases, the organic solvents most often used were chlorinated solvents (also known hazardous volatile organic compounds), such as methylene chloride or chloroform, and the azo coupling reaction under these biphasic conditions proceeded only if in the presence of a phase-transfer catalyst. Also, the equipment requirement here was again a multi-reactor set-up.
It remains a problem to be solved to have an easy, efficient and cost-effective, while more environmentally responsible, synthesis of azo compounds from hydrophobic couplers.